If avr-libc wants to hook in, it should install it's addendum to stdfix.h. There are basically two ways to do it:

1) Install it as stdfix-avrlibc.h and stdfix.h will pick
it up (preferred).

2) Use the GNU extension #include_next <stdfix.h> to include
avr-libc's stdfix.h

Stuff that avr-gcc's stdfix.h provides are basic definitions from

7.18a.1 Introduction (sat, fract, accum)

7.18a.3 Precision macros (IBIT, FBIT, MN, MAX, EPSILON, ...)

and maybe more stuff that's easy to implement like

2.1.7.4 bits/fx/

2.1.7.5 /fx/bits

more such basic stuff like absfx, roundfx that could

be provided with built-ins.

It would already help if you provide a dummy file that does nothing. Without even a dummy, the above includes will barf, of course. But with them, you can hook in new stuff at any time and at your option.